As logic integrated circuits (ICs) have migrated to lower working voltages in the search for lower power consumption and higher operating frequencies, and as overall system sizes have continued to decrease, IC power supply designs with smaller size and higher efficiency are in demand. Switch mode power supplies, or DC to DC converters, as they are sometimes called, are frequently used in IC circuits such as digital signal processors (DSPs) and mixed signal analog circuits, etc., to efficiently convert an input supply voltage to voltage levels appropriate for internal circuitry as well as external circuitry that the IC is coupled to. For instance, a 2.8 volt supply voltage provided to a BiCMOS IC may need to be increased internally to 5.0 volts to operate internal CMOS circuitry. As appliances and circuit designs have continued to decrease in size, the need for smaller and more efficient IC switch mode power converters has increased.
FIG. 1 illustrates a block diagram of a buck topology switch mode power converter 10 of the prior art. Control circuit X.sub.3 alternately turns on gate drivers X.sub.1 and X.sub.2. When driver X.sub.2 is off, the gate of Field Effect Transistor (FET) MN.sub.2 is connected to ground 12. FETs MN.sub.1 and MN.sub.2 are typically N-channel MOSFETs, for example. When X.sub.2 is on, the gate of FET MN.sub.2 is tied to V.sub.driver, turning on FET MN.sub.2. When driver X.sub.1 is off, FET MN.sub.1 is connected to ground 12, turning off FET MN.sub.1. In some prior art buck topology converters, when driver X.sub.1 is off, FET MN.sub.1 is connected to the source of FET MN.sub.1, which has the same effect as the circuit topology shown.
When driver X.sub.1 is on, the gate of FET MN.sub.1 is connected to the upper plate 14 of C.sub.boot. Capacitor C.sub.boot is adapted to have some positive voltage stored on it, V.sub.boot. The lower plate 16 of capacitor C.sub.boot is coupled to the source of FET MN.sub.1, and by this means, the gate of FET MN.sub.1 is pulled up to voltage V.sub.boot above the source of FET MN.sub.1. Therefore, FET MN.sub.1 is turned on, even as the source of FET MN.sub.1 rises. Before FET MN.sub.1 is turned on, the source of FET MN.sub.1 is pulled to ground by FET MN.sub.2 : FET MN.sub.2 is then turned off and FET MN.sub.1 is turned on. As a result, the source of FET MN.sub.1 rises to the voltage V.sub.in and the gate of FET MN.sub.1 rises to (V.sub.in +V.sub.boot). The use of the "bootstrap capacitor" or boot capacitor C.sub.boot is a feedback technique which tends to improve linearity and input impedance of circuits operating over a wide range of input signals. Specifically, the boot capacitor C.sub.boot allows FET MN.sub.1 to be turned on without there being a permanent supply available which is high enough to hold FET MN.sub.1 on even when the source of FET MN.sub.1 is at V.sub.in.
When FET MN.sub.1 is turned off and FET MN.sub.2 is turned on again, the source of FET MN.sub.1 returns to ground; however, some charge has been taken off boot capacitor C.sub.boot, so its voltage V.sub.boot is lower than before. The voltage V.sub.boot on boot capacitor C.sub.boot needs to be restored to its previous value. In the prior art circuit 10 shown, the restoration of boot capacitor C.sub.boot is accomplished by diode D.sub.1 that is connected between voltage V.sub.driver and the upper plate 14 of boot capacitor C.sub.boot.
FIG. 2 is a block diagram of a charge pump of the prior art typically used to generate the voltage V.sub.driver for the circuit 10 shown in FIG. 1 from supply voltage V.sub.in. The capacitor C.sub.pump is first charged to voltage V.sub.in by closing switches S.sub.1 and S.sub.3, with switches S.sub.2 and S.sub.4 open. Next, switch S.sub.1 and S.sub.3 are opened, and switches S.sub.2 and S.sub.4 are closed. Some of the charge stored in capacitor C.sub.pump is pumped into capacitor C.sub.driver. When switches S.sub.2 and S.sub.4 are closed, the voltage at node A is slightly higher than the voltage at V.sub.driver, which is caused by the voltage drop across switch S.sub.4, created by the current flowing from capacitor C.sub.pump to capacitor C.sub.driver.